The term “variable displacement engine” is used herein to refer to an engine having cylinder disablement, that is to say an engine of which one or more of the cylinders may selectively be prevented from firing and from contributing to the output power. Cylinder disablement can be performed in a variety of ways in both spark ignited and diesel engines by controlling the fueling and in some cases by also modifying the valve timing. The manner in which the displacement of the engine is varied is not of fundamental importance to the present invention and need not be described herein in detail.
When a vehicle is equipped with a variable displacement engine, there are only specific regions of the speed/load map within which it is more efficient to operate with cylinder disablement.
FIG. 1 of the accompanying drawings is a graph of engine speed versus engine torque on which there is marked out a range within which operating in a cylinder disablement (VDE) mode is more efficient. Within the range in which the VDE mode is more efficient, the fuel efficiency improvement is not constant and, as shown in FIG. 1, the improvement in efficiency reduces with increasing torque demand.
Under dynamic driving conditions, especially when the engine is operating around the threshold for the VDE deactivation mode, i.e. the perimeter of the range marked out in FIG. 1, frequent switching may occur between the VDE and normal (i.e. all cylinders firing) modes. Aside from such frequent mode changes being disconcerting to the driver, they subject the engine components to wear and affect the durability of the engine. The counter measures needed to improve robustness would have an impact on the system cost.